This question, Difference between "fluid" and "liquid", was closed as an exact duplicate. This one, "In time" or "On time"?, was merged. Why is this? Both questions had two answers with no down votes. In both cases, the first question (the one not closed) also already had an accepted answer. If these two factors obviously played no part in it, why did the moderator (both were closed or merged by RegDwight АΑA♦) choose differently on the two?
Any question closed as a duplicate of another is a potential merge candidate.
However, while a closed question can still be reopened (for whatever reason), a merge is irreversible. Consequently, I will usually give people time to dispute the decision — especially if I closed the dupe unilaterally, as was the case with the fluid vs. liquid question. This, in turn, can mean that I forget all about it — especially if people actually don't dispute the decision, as was the case with the fluid vs. liquid question.
Now that it's been closed for five months, and seeing how both answers work fine on the older question, I went ahead and performed the merge. (Sometimes answers do have to be changed accordingly, e.g. if the two OPs use different example sentences. If I don't have the time for that, again, I will postpone the merge.)
If you see another question that has been closed as a dupe for months with nobody appealing against it, flag it for mod attention.